The present invention relates to methods for producing powdered coating masses by converting polyesters containing hydroxyl groups with isophorondiisocyanate adducts partially blocked with .epsilon.-caprolactam. The invention also relates to the products produced by said methods which are used as coating materials.
It is known, e.g. from German Pat. No. 55 820, to apply powdered mixtures of high melting polyesters containing hydroxyl groups and polyisocyanates whose isocyanate groups are capped by phenol, by electrostatic spraying onto substrates and then to heat them to form hardened coatings. The disadvantages of this method include evolution of strongly malodorous phenol, frequent insufficient reactivity in the presence of catalysts, and process disturbances, such as bubble formation, for example. According to the teaching of German Offenlegungsschrift No. 19 57 483 these disadvantages are avoided by using .epsilon.-caprolactam capped polyisocyanates instead of the phenol-capped compounds mentioned above.
German Offenlegungsschrift No. 21 15 777 also describes powdered coating materials based on polyesters containing hydroxyl groups and .epsilon.-caprolactam blocked polyisocyanates which are characterized by the fact that they contain, as .epsilon.-caprolactam blocked polyisocyanates, such polyisocyanates as those based on 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate (subsequently designated IPDI or isophoronediisocyanate) or the products of its reaction with polyols, wherein the polyisocyanates and the polyol are used in such proportion that the resulting product has at least two NCO-groups.
The mixture of polyesters containing hydroxyl groups and polyisocyanates blocked with .epsilon.-caprolactam described in the documents mentioned above are cured at 180.degree.-200.degree. C. within about 30 minutes. There is great interest in .epsilon.-caprolactam blocked polyisocyanates in the production of powder hardeners for polyisocyanates which definitely unblock below 200.degree. C. The unblocking temperature can be lowered by the use of catalysts such as tertiary amines; however the result is cured films which are more susceptible to hydrolysis and less resistant to oxidation.
The surprising and welcome discovery has now been made that the hardening temperature mentioned in the German Offlegungsschrift can be lowered about 20.degree.-30.degree. C. by using in place of the blocked isophoronediisocyanate adducts or isophoronediisocyanate/glycol adducts, isophoronediisocyanate adducts which are only partially blocked with .epsilon.-caprolactam and which contain per mole more than 0.1 but no more than 1.0 moles of free NCO-groups.
These isophoronediisocyanate adducts, partially blocked with .epsilon.-caprolactam, which are described in futher detail hereinbelow, are compounds which are obtained by partial .epsilon.-caprolactam blocking of isophoronediisocyanate adducts which contain urethane, biuret, isocyanurate, or carbodiimide groups and have an NCO-content of 17-35%. These isophoronediisocyanate adducts which are only partially blocked with .epsilon.-caprolactam are solid and melt at 65.degree.-150.degree. C.